split face along edge
This method uses only built in functionality and does keep your UV coordinates in place.

First move the two inner edges so you will get a distinct intersections. Then select one of the intersecting faces and use the "Intersect (Knife)" tool (in the face menu). This will create an edge at the intersection between the two faces. It will also create some extra vertices that you need to dissolve.



Remove the overhanging triangle faces and delete them. Move the inner edges back to their position. Select the corner vertices and Remove Doubles.